


of all the wishes in the sea

by hovercraft



Category: Fate/Grand Order, Fate/stay night & Related Fandoms
Genre: M/M, Pining, mermaid au, no beta we die like men
Language: English
Status: Completed
Published: 2020-07-03
Updated: 2020-07-03
Packaged: 2021-03-05 01:27:07
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,092
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/25056163
Author URL: https://archiveofourown.org/users/hovercraft/pseuds/hovercraft
Summary: Cu holds only one memory dear to him, that memory disappeared in his childhood. He chases it to the ends of the Earth.Birthday fic for a friend.(Cu Alter/Diarmuid, with a little splash of Gil/Arthur)
Relationships: Cú Chulainn Alter | Berserker/Diarmuid Ua Duibhne | Lancer, Gilgamesh | Archer/Arthur Pendragon | Saber
Comments: 2
Kudos: 47





	of all the wishes in the sea

**Author's Note:**

> This is for my friend, Rae!! I wrote two versions of this fic, one with regular Dia and one with her AU Dia. This is the one with OG Dia. -w-b
> 
> I'm not the first to write a mermaid AU featuring any of these characters but it is an AU we talked about a lot in Discord DMs, so I ended up writing it for her birthday, wahey.

“Sétanta! I want this deck clean! You can’t run off until it’s done!”

Cu thought to himself _, it’ll get done when it gets done_ , not giving much consideration to the captain’s words. It wasn’t like he was going anywhere anyway; this was the open ocean. If he wanted to get away from his captain’s orders, he’d have to swim. In either case, he left the mop in its bucket and stared out across the open ocean, trying to spot something interesting in the distance. It was night, so it was a little more difficult than usual, but they said these waters held unique legends. He wanted to be among the rare privileged men able to see a mermaid out at sea.

He just hadn’t anticipated it would be tonight.

The sky was a cool, clear black, with the ocean reflecting it and the moon off its shimmering waves. All he wanted to do in that exact moment was relax.

The boat was anchored for the moment, so he didn’t have any fear—he climbed down the side of the boat to get a good look at the ocean, hanging from the ropes over the side. A dangerous feat, one Scathach had warned him about before, but he didn’t anticipate seeing someone swimming just beneath the current of the water.

It was someone who had been watching him this whole time.

The soft, eerie tone of their voice was alluring, but not threatening. Cu got down as close as he could to the water. He extended a hand, wondering first if this was someone who had fallen overboard.

The hand that grabbed his was covered in shimmering emerald green scales, and he nearly jerked his hand away in shock if it hadn’t been for the fact that this mercreature pulled him into the water after them. With his hand on one of the ropes, he swam back up to the surface and gasped for air, hanging onto the boat for dear life. Nothing could prepare him for what was in front of him when he wiped the saltwater from his eyes.

A pretty boy.

A merman, and here he’d been warned never to follow the voice of one. They eat sailors, don’t they? Lure them to their deaths? This one didn’t look so… threatening. He touched his cheek like Cu was a rare commodity, and he returned the favor. It must have tickled, because he laughed and splashed his face with water once Cu reached over and pinched his cheek. Gods above, his voice was melodious, beautiful in every way it spoke.

“Who are you?” Cu asked.

“Diarmuid Ua Duibhne,” He whispered to him. “What’s yours?”

“Sé—no, it’s Cu Chulainn.”

It felt like an intimate exchange, in hushed tones so Scathach wouldn’t hear them. Diarmuid asked him idle questions about humanity, which he answered to the best of his ability—books didn’t survive underwater, so knowledge of the human world was scarce. From that night on, Diarmuid would tail the ship and meet with Cu every night, on the lowest steps of the ship, as he shared things about the human world with him. He showed him a flame from the torches on the ship, he showed him maps, and he never pressed too hard on what the world of mercreatures was like, sensing it was a secret best kept.

It was the most magical thing Cu would ever experience in his youth. Once he went to shore, he would never see him again, but he would always be kept safe in his memory. As his life went from peaceful to chaotic, bad to worse, he would still _always_ remember him.

\--

Those were better times.

When he was young and carefree, and now he’s older and carefree but not in the good way, no—he doesn’t care about anything or anyone. He cannot bring himself to care, now that he helms his once master’s ship and plunders the seas for all they’re worth. No matter how big the cache of gold he finds, he just says ‘bring it back to the ship’ and says nothing more. He is the wealthiest man on the sea barring Gilgamesh, and he has all the evidence to show it.

There is one memory that persistently stands out to him. The merman he met when he was a child.

Finding a merman or mermaid in the sea is like finding a needle in a haystack, but that’s just the sort of challenge Cu longs for. He tells his men the kind of person they’re after, and occasionally, they bring up the wrong one—and Cu won’t allow them to put it in the tank to sell later. That tank is reserved for one merman alone.

He knows Diarmuid will find him before he finds him. It’s just an instinct he has.

Sure enough, one night when the moon is high in the sky, he can hear an old Irish song across the ocean in a voice he knows is too familiar to ignore. Shrugging off his captain’s jacket and striding out across his deck, he climbs down the side of the boat once it’s been anchored and meets the man who haunts his dreams.

He has grown up, with sharp teeth and flowing black hair, but he would never mistake him for anyone else. The beauty mark beneath his eye, the glowing golden gaze… he reaches out for Diarmuid’s hand, and he pulls him into the water. His men are watching from the ship deck, and it’s the perfect time for a mutiny, but it doesn’t happen. Cu talks to him in hushed tones and asks Diarmuid why it took him so long to appear. He responds—it was always the wrong place, wrong time. Cu tells him to come with him. He looks unsure…

He wasn’t asking.

Hauling him over one arm and climbing up the ropes with the other, he carefully holds onto Diarmuid while he climbs up. His men hoot and holler, but he tells them to shut up as he carries him in his arms, to the captain’s room, where a tank has been built just for him. He is confused—understandably so—why he’d want to imprison him. Cu tells him it’s not imprisonment—he just needs to be here for a little while. He needs to see if his fascination will go away once he finally has him, and if it does, he’ll let him go.

\--

It doesn’t go away.

Diarmuid is the first thing he sees when he wakes up, and the last thing he sees when he goes to bed.

He is always watching him, listening to his song, finding excuses to go back to the tank.

Eventually, he has enough. Surely, if he lets him go, he will get rid of this fascination once and for all, but he can’t, and it makes him so _weak_. He cannot let him go so easily, but he is not a king of a hoard, like Gilgamesh is—he is a king on a wasteland, with nothing to call his own, wild and free—he cannot let this happen. He takes him out of the tank and offers to set him free.

“No.”

“…?”

Surely, he must be hearing things.

“You’re hurting, Cu. My dearest and oldest friend.”

“…”

“I want to stay with you until you’re feeling better.”

There is no ‘better’, he thinks. This is his ‘normal’.

But without Diarmuid’s permission to free him, he doesn’t have the strength to let him go, but he can’t stay in a tank forever.

\--

It’s one of Gilgamesh’s contacts who helps him.

A man named Merlin knows the trick to changing a mercreature into a human, and though the process is painful, Diarmuid agrees to it.

With no gills, he must learn how to breathe. With legs, he must learn how to walk.

Cu commits himself to teaching him.

He was beautiful and radiant as a mermaid and is just as much so as a human, even as he wobbles and has to cling onto him to walk as he balances on his feet. He wonders if he’s robbed him of everything. Eventually, Diarmuid sits down to talk to him about it.

He was an outcast underwater, once charmed to love someone he didn’t love and exiled from his proudest position as a knight. He escaped and had been wandering the water freely ever since. Escaping the water was like escaping his pain. Diarmuid asks Cu if he thinks he’s just using him as an excuse to get away.

Cu asks him, “What does it matter?”

Diarmuid doesn’t answer.

Perhaps Diarmuid had chased Cu just as much as he chased him, a fond memory in a sea of unpleasant ones. A guiding light, a lighthouse on the shore.

Cu realizes he should apologize for the way he treated Diarmuid before, and he does.

He tells him it’s fine. Just so long as Cu knows he does want to return to the sea, someday.

Cu feels his insides freeze up but nods. It’s like if someone asked him if he wanted to drown for the rest of his life, he cannot ask him to stay by his side forever.

\--

Walking and breathing is not the extent of his lessons. He can’t keep his hands off Diarmuid for very long.

Though it’s painful at first, Diarmuid quickly realizes why his legs are so useful. When Cu makes love to him, it’s a different sensation than any he’s ever known, and within it, he can sense his quiet desperation. ‘Don’t leave me,’ though unspoken and never going to pass through his lips, is loud and clear. It seems too many of Cu’s life lessons and people within them were transient, gone as soon as they came, and the experience changed him. Some to death, others to time, they just … left, and he was supposed to sort that out by mending himself.

It’s that same possessive claiming that reminds Diarmuid of what is at stake. What he stands to lose in losing him. It’s things like this that whittle away his willpower to go back to the ocean.

Is it bad? Not really. Is it right? Not at all. But is it fair? Never.

\--

Diarmuid is surprised to meet the king of the seas on two legs. Arthur Pendragon found himself in the same predicament as him. Captured aboard a ship by a king on land, though their history together went far further than that—or at least, that was the case at first. Diarmuid sees how tenderly he looks at Gilgamesh, how he knows he has duties underwater but still cannot give up the love of his life. They share a history that neither will talk about, and while Gilgamesh is cruel and cunning, it seems he reserves some tenderness in his heart for Arthur.

“What will you do about it?” Diarmuid asks.

“I don’t know,” Arthur is nothing but honest. “He knows I am a king, but at the same time, I’ve never been more free and alive than on land. Here I am nothing but loved, when underwater, all my time was spent fighting for territory. He says that human beings are allowed to be selfish, so now that I am one, I can make the choice…”

Guilt is evident in his voice, but Diarmuid persists. “You had an heir, didn’t you?”

“Yes. Mordred.”

“Then let them take care of things. Maybe you’ll only need to return the once to pass on your crown?”

Arthur flinches, knowing that’s what he’s afraid of. He’s not a prince, he’s a king, he cannot shirk his job for the sake of love. Diarmuid can, though, as much as he longs for the pride of Fianna.

\--

Diarmuid eventually decides that the ocean is no longer worth it.

Arthur might have decided to go back perhaps temporarily, with Gilgamesh looking ruefully at the sea, wondering if he’d ever return or leave him with a broken heart. Diarmuid couldn’t do that to Cu. His loyalties had somehow changed over time.

He finds Cu on his ship and sits in his lap, arms wrapped around his shoulders. It takes him a moment, but he holds onto Diarmuid’s back with one of his heavy hands.

“What will you do?” He asks.

“My home is wherever you are.”

“Hm.” He wonders if he forced Dia into making this choice, and therefore, could it even be called one?

“Cu…”

“…” He murmurs something.

“What is it?”

“Thank you.”

It seems like that’s all he can say.


End file.
